Electrical cables include a conductor core and typically include an outer jacket or sheath. The term “sheath,” as used herein and throughout the specification and claims, is defined to mean the outermost protective jacket or covering surrounding a conductor core, whether of a single type material or multiple layers of the same or different material. The conductor core may typically be, for example, a single metal wire, multiple small wires twisted together to make a “stranded” cable, or multiple insulated wires or other type electrical conductors acting together to serve a particular function (e.g., three-phase connection). The sheath may comprise one or more layers of polymeric or other material to provide physical, mechanical, electrical insulating and/or chemical protection for the underlying cable components. For the purpose of type THHN cable of the present invention, the exterior portion of the sheath is of nylon. Specifically, type THHN cable comprises a conductor core of a single solid or stranded conductor, surrounded by a layer of polyvinyl chloride (PVC) electrical insulation, covered by an outer layer of nylon.
Installation of electrical cable often requires that it be pulled through tight spaces or small openings in, and in engagement with, narrow conduits, raceways, cabletrays, or passageways in rafters or joists. This becomes problematic since the exterior surface of the cable sheath normally has a high coefficient of friction, therefore requiring a large pulling force. Moreover, installation parameters include maximum allowable cable pulling tension and /or sidewall pressure limits. Exceeding these limits can result in degradation of the cable, physical damage and inferior installation.
To overcome this problem, the general industry practice has been to coat the exterior surface of the cable sheath with a pulling lubricant at the job site in order to reduce the coefficient of friction between this surface and the conduit walls or like surfaces, typically using vaselines or lubricants produced specifically, and well known in the industry for such purpose, such as Yellow 77® (hereinafter, “Y 77”). The term “pulling lubricant,” as used herein and throughout the specification and claims, is defined to mean lubricating material which sufficiently reduces the coefficient of friction of the exterior surface of the sheath of the cable to facilitate the pulling of the cable.
The aforementioned industry practice of applying a pulling lubricant like Y 77 to the finished cable at the job site poses problems, principally due to the additional time, expense and manpower required to lubricate the finished cable surface at the job site as well as to clean up after the lubricating process is completed. Alternative solutions have been tried but are generally unsuccessful, including the extrusion of a lubricant layer over the extruded polymeric sheath during the manufacturing of the cable, or the application of granules of material to the still-hot sheath during the extrusion process, which granules are designed to become detached when the cable is pulled through the duct. However, these solutions not only require major alterations of the manufacturing line, but result in a loss in manufacturing time, increased economic costs, and undesirable fluctuations in the geometrical dimensions of the cable sheaths.
It is also important to an understanding of the present invention to know the difference between what are referred to as “pulling lubricants” and what are “processing lubricants.” A pulling lubricant is a lubricant that appears at the outside surface of the sheath of the cable and is effective to lower the surface coefficient of friction such as to reduce the force necessary to pull the cable along or through building surfaces or enclosures. A processing lubricant is lubricating material that is used to facilitate the cable manufacturing process, such as the flow of polymer chains during any polymer compounding as well as during the extrusion processes while the polymer is in its molten or melt phase. Cable manufacturers have long used processing lubricants, such as stearic acid or ethylene bis-stearamide wax, as a minor component of the polymeric compound from which the cable sheath is formed. Because a processing lubricant is normally not effective except when the polymer is in this melt phase, the effect of a processing lubricant is essentially non-existent in the final hardened polymer sheath of the cable. Even where there may be an excessive amount of the processing lubricant, a separate pulling lubricant would still be required to sufficiently reduce the cable sheaths' exterior surface coefficient of friction in order to minimize the pulling force necessary to install the cable.
Accordingly, there has been a long-felt need for an effective method of providing a pulling lubricant at the exterior surface of the finished cable, and particularly the finished THHN cable, which is effective to reduce the cable surface coefficient of friction and minimize the required installation pulling force, without incurring the inconvenience and time-consuming operation and expense associated with the application of the pulling lubricant at the installation site, nor significantly increasing the complexity and cost of the manufacturing process, nor undesirably altering the geometrical characteristics of the cable sheaths.